Book 1 Chapter 6.7 `dev_major`: `ray_color` function

[LollipopFt]

raytracing.github.io/books/RayTracingInOneWeekend.html

Lines 1244 to 1246 in 806ab49

	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
	auto a = 0.5*(unit_direction.y() + 1.0);
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++

as a follow-up to #1059 , since a is already initialised, there should be no change to the line linked above and thus it should not be highlighted.